It’s important to have your timing belt looked at when your car has gone about 100,000 miles. This is a good rule of thumb but always consult each manufacturer for the the mileage interval that they recommend.
Plain & simple, don’t take the chance of your timing belt breaking or “jumping” a tooth. Although many engines are considered “non interference”, meaning that if the belt breaks the valves will not come in to contact or interfere with the pistons, causing engine damage. Even if they are non interference, if the engine is revving high enough the valves could “float” and still come in to contact with the pistons.
The other problem that may arise would be if the belt did not completely break but “jumps” a tooth or two. This will cause the valve timing to be off and can be a very difficult problem to diagnose. It can cause a host of issues including poor fuel economy, drivability concerns and engine lights.
When replacing the belt be sure to check the water pump and idler pulleys and tensioner. It would not be good to have to redo a job six months later because a pump started leaking or a pulley started making noise. Or even worse, the belt breaks due to a seized pulley. Yes, this does happen.
In summary, it is not much fun to have an expense on your car that gives you nothing tangible in return. On the other hand, you can plan and budget for the cost of your timing belt . An engine replacement is never a planned expense.